Heart stimulator with evoked response detector and an arrangement for determining the stimulation threshold

ABSTRACT

A heart stimulator has a pulse generator which emits stimulation pulses of different amplitudes, which are delivered to a patient&#39;s heart via a lead connected to the pulse generator. The pulse generator is controlled by a control unit, in a procedure for determining a stimulation threshold value, to emit stimulation pulses of successively unidirectionally changing amplitudes. Each stimulation pulse is followed by a test pulse having a predetermined high, constant amplitude delivered in a period corresponding to the refractory period after the preceding stimulation pulse. Each test pulse has an amplitude sufficient to capture a non-refractory heart. A measurement unit is connected to the electrode lead, and measures a signal picked up by the lead after each test pulse. The measured signals are supplied to a comparator, which compares the signals following the test pulses with each other. A significant change in the measured signal after the test pulses indicates that the amplitude of the stimulation pulses has passed the threshold value.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a heart stimulator having a pulsegenerator devised for producing stimulation pulses of varyingamplitudes, a lead being intended to be introduced into the heart of apatient and connected to the pulse generator for delivering stimulationpulses to the heart.

2. Description of the Prior Art

To reduce the energy consumption of heart stimulators an automaticthreshold search function, a so called AUTOCAPTURE™ function, is used tomaintain the energy of the stimulation pulses at a level just above thatwhich is needed to effectuate capture, cf. e.g. U.S. Pat. No. 5,458,623.A reliable detection of the evoked response, which then is necessary,is, however, not a simple matter, especially when it is desired to sensethe evoked response with the same electrode as the one delivering thestimulation pulse. The reason therefor resides in the fact that theevoked response potential is small in amplitude compared to the residualpolarization charge. The residual charge decays exponentially but tendsto dominate the evoked potential for several hundreds of millisecondsafter the stimulation. If the polarization is too high, it could bewrongly interpreted by the evoked response detector as a capture, i.e.contraction of the heart. The AUTOCAPTURE™ algorithm could then bymistake adjust the output amplitude of the stimulation pulse to a valuebelow the actual capture level, which will result in no capture. If theused pacing lead has significant polarization this could consequentlydisturb the AUTOCAPTURE™ function and result in loss of capture.

Several attempts have been made to solve the lead polarization problemsin connection with evoked response detection. One possibility is to uselow polarization leads. This is, however, not always possible.

Another method is described in U.S. Pat. No. 5,417,718, which disclosesa system for maintaining capture wherein electrical post-stimulus signalof the heart, following delivery of a stimulation pulse, is compared toa polarization template, determined during a capture verification test.A prescribed difference between the polarization template and thepost-stimulus signal indicates capture. Otherwise loss of capture ispresumed and the stimulation energy is increased a predetermined amountto obtain capture.

In U.S. Pat. No. 5,697,957 a method and an apparatus for extracting anevoked response component from a sensed cardiac signal by suppressingelectrode polarization are descsribed. An autocorrelation function isthen calculated according to an autocorrelation algorithm, and isapplied to the sensed cardiac signal. The autocorrelated signal thusobtained and the sensed cardiac signal are then normalized to each otherand a difference between these two normalized signals is formed, therebyextracting the evoked response component if present in the cardiacsignal.

In U.S. Pat. No. 5,741,312 a method and an apparatus are described todetermine stimulating threshold through delivery of pulse pairsconsisting of a first lower amplitude search pulse with variableamplitude and a second regular pacing pulse within 50–100 ms. Thresholdsearch is executed by incrementing the amplitude of the search pulseuntil an evoked response is detected. Alternatively the period fromregular pacing pulse to the T-wave is measured and capture on the searchpulse is determined as a sudden shortening of this interval. U.S. Pat.No. 5,741,312 further discusses methods to minimize polarization byoptimizing pulse paramers of a two- or triphasic pacing pulse.

There is mostly at least one significant slope in the bipolar measuredIEGM signal, which makes it possible to discriminate the evoked responsesignal from slowly varying signals such as polarization signals. Thus inU.S. Pat. No. 5,431,693 a method of verifying capture of the heart by acardiac pacemaker is described. Observing that the non-capture potentialis exponential in form and the evoked capture potential, while generallyexponential in form, has one or more small-amplitude perturbationssuperimposed on the exponential wave form, these perturbations areenhanced for ease of detection by processing the wave form signal bydifferentiation to form the second derivative of the evoked responsesignal for analysis for the evoked reponse detection.

Unipolar detection of evoked response signals is however not possible bythis technique. Abrupt slope changes or superimposed small-amplitudeperturbations are levelled out if the measurements are made over alonger distance from the electrode to the stimulator casing.

This is illustrated in FIG. 1 herein, which shows the unfilteredmeasured electrode signal picked up by a unipolar electrodeconfiguration, the upper curve in the figure, and a bipolar electrodeconfiguration, the lower curve in FIG. 1.

In co-pending United States Application filed simultaneously herewithand identified with International patent application No. PCT/SE99/01017)a new technique is described for solving the polarization problem inconnection with evoked response detection. This technique is not basedon any slope measurements on the sensed electrode signal, but ondetermination of the polarization signal for different stimulationamplitudes for then subtracting the polarization signal from the sensedelectrode signal to obtain the true evoked response signal. Thisdetermination of the polarization is based on the observations that theevoked response signal amplitude is fairly constant, independent of thestimulation pulse amplitude, whereas the electrode polarization isapproximately linearly dependent on the stimulation pulse amplitude fora constant pulse duration, cf. European Application 0906768.

The above mentioned manner of determining the polarization presumes thatthe stimulation threshold value is known.

SUMMARY OF THE INVENTION

An object of the present invention is therefore to provide a techniquefor determining the stimulation threshold value which is suitable to usein connection with the new way of determining the polarization asdescribed in the aforementioned co-pending patent application.

The above object is achieved in accordance with the principles of thepresent invention in a heart stimulator having a pulse generatoroperated by a control unit to emit stimulation pulses of differentamplitudes, which are delivered to a patient's heart via an electrodelead connected to the pulse generator. The pulse generator is operatedin a procedure to determine the stimulation threshold value to emitstimulation pulses of unidirectionally changing (i.e., decreasing orincreasing) amplitudes. Each stimulation pulse is followed by a testpulse having a predetermined high, constant amplitude which is deliveredin a period corresponding to the refractory period after the precedingstimulation pulse. The test pulse has an amplitude sufficient to capturea non-refractory heart. A measurement unit is connected to the electrodelead, and measures signals picked up by the electrode lead after eachtest pulse. The measured signals are provided to a comparator whichcompares the measured signals following the test pulses with each other.A significant change in the measured signal after the test pulsesindicates that the amplitude of the stimulation pulses has passed thethreshold value.

Thus with the heart stimulator according to the invention the averageamplitudes of the signals following each test pulse are determined, andas long as there is capture on the stimulation pulse, the amplitude ofthe polarization following the test pulse will show small variations.When the stimulation pulse no longer causes capture, there will be acapture on the test pulse and the amplitude of the following signalswill change significantly, thus indicating that the amplitude ofstimulation pulse has passed the threshold value. Thus, in this way thethreshold value is localized.

The stimulation threshold can be determined before or during thepolarization algorithm executed in the evoked response detectoraccording to the aforementioned co-pending International patentapplication No PCT/SE99/01017, which is an important advantage.

In an embodiment of the heart stimulator according to the invention acalculation unit is provided to calculate the average value of theamplitude of the electrode signal picked up after each test pulse, andthe comparator compares the average values with each other to detectsignificant changes in said average values for use in the determinationof threshold value. The measurement unit is preferably adapted to sampleand digitize the measured electrode signal during a predetermined timeinterval after the delivery of the test pulse and the calculation unitis adapted to calculate an average value of said samples. In this waysmall variations and other interferences in the measured electrodesignals are suppressed.

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the unfiltered electrode signal measured with a unipolarelectrode configuration in the upper curve, with the unfilteredelectrode signal measured with a bipolar electrode configuration in thelower curve.

FIG. 2 is a block diagram of a heart stimulator constructed andoperating in accordance with the principles of the present invention.

FIG. 3 is a block diagram of an embodiment of an evoked responsedetector used in the heart stimulator according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2 shows a block diagram of the principal layout of the heartstimulator according to the invention. The stimulator includes a pulsegenerator 2 which through a lead 6 is connected to the heart 8 of apatient. The pulse generator 2 is devised to produce stimulation pulsesof varying amplitudes which through the lead 6 are transferred to theheart 8. An evoked response detector 4 is also connected to the lead 6.The evoked response detector 4 includes a filter and measurement unit 10for measuring the electrode signal picked up by the lead 6.

The measured electrode signal is supplied to a calculating unit 16 andto a comparator 12 for comparing the measured electrode signals witheach other.

The filter and measurement unit 10 is disconnected by the switch 11 fromthe lead 6 during stimulation.

A timer 14 is provided for determining a time interval after thedelivery of the test pulse during which the electrode signal is measuredand stored. The measurement unit 10 is adapted to sample and digitizethe measured electrode signal during this time interval and thecalculation unit 16 calculates an average value of these samples. Thisaverage value is then supplied to the means 12 for use in the subsequentcomparison step.

As test pulse the ordinary backup pulse of the heart stimulator canpreferably be used.

FIG. 3 shows in more detail an embodiment of the evoked responsedetector used in the heart stimulator according to the invention. Theheart electrode signal picked up by the lead 6 in FIG. 2 is thensupplied to a highpass filter 20. An amplifier 22 and an A/D converter24 are provided for amplifying and A/D converting respectively thefiltered signal. A digital signal processor 26 measures, calculates andcompares the signals picked up by the lead 6.

Thus in the embodiment shown in FIG. 3 the algorithm for determining thestimulation threshold value is implemented in software by use of amicroprocessor. Instead of a microprocessor this algorithm can also beimplemented in random logic, which means realization by ordinary logicelement, that is logic gates.

The detector can also be implemented in the electronics of the heartstimulator according to the invention by use of switch capacitor (SC)technique. The algorithm is then implemented in SC technique, wheredifferent capacitors serve as memory elements for storing the differentelectrode potentials and SC-adding, subtracting and multiplying circuitsare used for performing the necessary calculations.

The function of the embodiment illustrated in FIGS. 2 and 3 is asfollows.

Stimulation pulses are delivered in a Vario cycle, i.e. the procedure isstarted with a stimulation pulse with a high amplitude and thenstimulation pulses of successively lower amplitudes are delivered. Aftereach stimulation pulse a test pulse of a high constant amplitude is sentout. This test pulse can preferably be the backup pulse of the heartstimulator in question. The average amplitude of the measured signalafter the test pulse is calculated as described above. As long as thedelivered stimulation pulses result in capture, the measured electrodesignal after the test pulse will be a pure polarization signal, and theaverage amplitude of these polarization signals will exhibit only smallvariations as the test pulses have a constant amplitude.

When the amplitude of the stimulation pulse is lowered such that captureis no longer obtained, the test pulse will give rise to capture and theaverage amplitude of the electrode signal measured after the test pulsewill change significantly and become much more negative than theprevious signal amplitudes. Thus, this change in the measured signalamplitude indicates that the threshold is passed and the threshold valuedetermined.

If the lowest possible stimulation amplitude is reached, e.g. 0.3 V, andno loss of capture is detected, the average electrode signal followingthe 0.3 V stimulation pulse is measured and compared with apredetermined value. There are three possible situations explaining thefact that no loss of capture is found, namely

-   1. The stimulation threshold is below 0.3 V and the absolute average    value of the measured signal is larger than the mentioned    predetermined value.-   2. The stimulation threshold is below 0.3 V and the absolute average    value of the measured electrode signal is smaller than the mentioned    predetermined value.-   3. The stimulation threshold is above 0.3 V, which consequently    results in the loss of capture when stimulating with a 0.3 V    stimulation pulse, but the absolute average value of the measured    electrode signal is smaller than the mentioned predetermined value    and therefor cannot be detected after the test pulse.

If the threshold is found and is above 0.3 V or if the situationaccording to point 1 prevails, the polarization signal POl_(step) forthe voltage step of the changing stimulation pulse amplitude can becalculated, either from the cycle of successively changing stimulationpulses already carried through, if the measured electrode signalsfollowing the stimulation pulse have been stored, or by starting a newsuch cycle running down to the threshold amplitude plus one voltagestep.

If the threshold is not found and the situation according to point 1above is not fulfilled, the explanations according to points 2 or 3 mustbe correct. In that case the measured electrode signal is too low tomake a reliable detection of evoked response possible and if the heartstimulator in question is provided with an AUTOCAPTURE™ function it mustnot be activated.

As an alternative to the above described procedure of successivelylowering the stimulation amplitude from a high starting amplitude (Variocycle), the procedure for determining the threshold can be as follows.

The cycle is started by stimulating a predetermined number of times witha high stimulation amplitude, e.g. 4.5 V, followed by a test pulse equalto a backup pulse of 4.5 V. Then the stimulation amplitude is changed tothe lowest possible stimulation amplitude, e.g. 0.3 V, if 0.3 V is thevoltage step for the heart stimulator in question, followed by backuppulses of 4.5 V. The average amplitude of the measured polarizationsignals following the backup pulses are then compared and it is decidedwhether there was a capture or not after the 0.3 V stimulation pulsesaccording to the above stated criteria. If there was capture,stimulation is performed with stimulation pulses of 4.5 V and then 0.6 V(0.3 V+0.3 V). From the measured electrode signals after the stimulationpulses, which resulted in captures, the POl_(step) signal is calculated.

If there was not a capture for a stimulation amplitude of 0.3 V, thelower stimulation amplitude is increased with the voltage step of 0.3and the procedure is repeated until the threshold value is reached.

If the stimulation threshold value is above a predetermined value, forboth the above described threshold searching methods, the POl_(step)signal can be directly calculatead from the polarization signalsmeasured for stimulation amplitudes below the threshold value.

Although modifications and changes may be suggested by those skilled inthe art, it is the intention of the inventors to embody within thepatent warranted hereon all changes and modifications as reasonably andproperly come within the scope of their contribution to the art.

1. A heart stimulator comprising: a pulse generator which emitsstimulation pulses, each having an amplitude; a lead connected to saidpulse generator and adapted for introduction into a patient fordelivering said stimulation pulses to the patient's heart; a controlunit connected to said pulse generator for controlling said pulsegenerator, in a procedure for determining a stimulation threshold value,to emit stimulation pulses of successively unidirectionally changingamplitudes, and a test pulse following each stimulation pulse, each testpulse having a predetermined high, constant amplitude and beingdelivered in a refractory period after the preceding stimulation pulse,the amplitude of each test pulse being sufficient to capture anon-refractory heart; a measurement unit connected to said electrodelead for measuring exclusively an amplitude of electrode signals pickedup by said lead after each test pulse, as measured signals; and acomparator supplied with said measured signals for comparing therespective amplitudes of said measured signals following each test pulsewith each other, and, if a significant change in said amplitude of saidmeasured signals after test pulses occurs, said comparator emitting asignal indicating that the amplitude of the stimulation pulses haspassed said stimulation threshold value.
 2. A heart stimulator asclaimed in claim 1 wherein said control unit controls said pulsegenerator to emit stimulation pulses of decreasing amplitudes.
 3. Aheart stimulator as claimed in claim 1 wherein said control unitcontrols said pulse generator to emit stimulation pulses of increasingamplitudes.
 4. A heart stimulator as claimed in claim 1 wherein saidcontrol unit controls said pulse generator to deliver stimulation pulsesof successively lower amplitudes, beginning with a high pulse amplitudeabove said stimulation threshold value and wherein said comparator emitssaid signal if a significant change occurs in the respective amplitudesof said measured electrode signals picked via said lead after said testpulse between two consecutive stimulation pulses, said signal indicatingthat the amplitude of the stimulation pulses has passed below saidthreshold value.
 5. A heart stimulator as claimed in claim 1 whereinsaid control unit controls said pulse generator to emit stimulationpulses respectively of first and second different amplitudes, said firstamplitude being equal to a highest available stimulation amplitude andsaid second amplitude being equal to a lowest stimulation amplitude, andwherein said comparator determines if said threshold value is betweensaid first and second amplitudes and, if so, said control unit operatessaid pulse generator to emit said stimulation pulses with the loweststimulation amplitude being successive increased by a predetermined stepuntil said threshold value is no longer found by said comparator to bebetween said stimulation pulses of first and second amplitudes.
 6. Aheart stimulator as claimed in claim 1 further comprising a calculatingunit connected between said measuring unit and said comparator, saidcalculating unit calculating an average value of the amplitude of therespective measured signals after each test pulse, and supplying saidaverage value to said comparator for use in determining said thresholdvalue.
 7. A heart stimulator as claimed in claim 6 wherein saidmeasurement unit samples and digitizes said measured signals during apredetermined time interval after delivery of said test pulse, andwherein said calculating unit calculates said average value from saidsamples.
 8. A heart stimulator as claimed in claim 1 wherein saidcontrol unit controls said pulse generator to emit said test pulse as abackup pulse for said pulse generator, having an amplitude ofapproximately 4.5 V.